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March 26 (Reuters) - Money spent on online advertising hasrisen rapidly, a leading forecaster said, despite talk aboutfirms trimming digital ad budgets as they question the medium'seffectiveness.

Zenith, owned by France's Publicis, forecast thatadvertisers would spend 40.2 percent of their budgets on onlinecampaigns this year, higher than 37.6 percent in 2017.

"For many consumers, checking their mobile devices forsocial media has become a regular, ingrained habit, while socialmedia ads blend seamlessly into their mobile app newsfeeds,"Zenith said in a statement.

Mark Zuckerberg's Facebook is often credited withspearheading the social media revolution that has grippedbillions of users globally.

However, the world's largest social media network facesgovernment scrutiny in Europe and the United States followingallegations by a whistleblower that British consultancyCambridge Analytica improperly accessed users' information.

Global advertising expenditure is forecast to expand by 4.6percent in 2018 to $579 billion, helped by improved economicgrowth in China and Argentina.

The new projection marks the biggest quarterly upgrade sinceMarch 2011 by Zenith, which had forecast 4.1 percent growth inDecember.

Zenith added that it expected advertising expenditure togrow more slowly than the global economy as a whole out to 2020.

Zenith forecast that global advertising spending would riseby $77 billion between 2017 and 2020.

The United States, the world's largest economy, wouldcontribute the most to this additional outlay - 26 percent, withChina ranked second.

A notable development in China is that television has foughtback against strong competition from online video and no longerloses ad spend, which it did in 2014, 2015 and 2017, Zenithsaid.

In the Middle East and North Africa, a fall in oil prices in2014 prompted advertisers to pare budgets in anticipation oflower consumer demand. Political turmoil and conflict haveworsened, further shaking advertisers' confidence in the region,Zenith said.

The forecaster estimates a 6.2 percent fall in ad spend thisyear in the region.

Zenith forecast 8.8 percent annual growth to 2020 in EasternEurope and Central Asia, which were hurt by the Ukraine conflictin 2014, foreign sanctions on Russia and the drop in oil prices.

(Reporting by Noor Zainab Hussain in Bengaluru; Editing by DaleHudson)